The invention relates to a device for determining local absorption differences a cross-sectional slice of in an object. The device comprises an X-ray source for generating a flat, fan-shaped X-ray beam which irradiates the object from different directions during an examination. The device also comprises an X-ray detector which is directed towards the X-ray source and which comprises a plurality of plate-shaped collimating elements which are directed towards the X-ray source. These collimating elements may be accommodated in a separate collimator arranged in front of the detector, or they may be arranged inside the detector.
A form of detector assembly which is often used in computer tomography devices comprises a gas-filled envelope in which a large number of plate-shaped electrodes defined individual ionization chambers. In such a detector assembly, the electrodes may contain a radiation-absorbing material and may even act as a collimating element themselves. Hereinafter, a detector having a separate collimator with a plurality of radiation absorbing collimator plates which are directed toward the X-ray source will be described in detail. However, it is not intended that the general scope of the invention should be restricted to such a detector.
A device of the kind described above is particularly suitable for medical X-ray diagnostics. During an examination, a sectional region of the body of a patient is irradiated from different directions by means of the flat, fan-shaped X-ray beam; locally transmitted radiation is measured. From the measurement data thus obtained, a computer calculates the density distribution in the body section of the patient situated in the irradiated region. This distribution is subsequently displayed, for example, on a television monitor.
In U.S. Pat. No. 4,051,379, a device is described in which an X-ray source, an X-ray detector, and a scattered radiation collimator are all rigidly connected to a rotatable support. The support has a central aperture for accommodating an object to be examined. The X-ray source is arranged on one side of the central aperture, and the X-ray detector and the scattered radiation collimator are arranged on the other side thereof.
During an examination, the rotatable support is rotated about the central aperture so that an object arranged in the aperture is irradiated from different directions. Radiation transmitted by the object is measured by means of the X-ray detector. The detector comprises a large number of individual elemental ionization chambers which are arranged on an arc of a circle and which are defined by plate-shaped electrodes which are directed towards the X-ray source. Collimator plates of the scattered radiation collimator are arranged along the respective lines of projection, of the plate-shaped electrodes towards the X-ray source. The collimator plates are thus directed, like the electrodes, toward the X-ray source.
In U.S. Pat. No. 4,093,863, a device is described in which an X-ray source, an X-ray detector, and a scattered radiation collimator are not rigidly connected to a single rotatable support. Instead, the X-ray source is mounted on a first rotatable support, while the X-ray detector and the scattered radiation collimator are mounted on a second rotatable support.
The X-ray detector in this device comprises a closed array of ionization chambers which are arranged along part of the circumference of a circle. The chambers are defined by plate-shaped electrodes which are directed towards the center of the circle. The rotatable supports are linked so that the center of the circle always coincides with the X-ray source during an examination. Collimator plates are arranged to extend along the respective lines of projection of the plate-shaped electrodes toward the source, so that they are directed, like the electrodes, toward the X-ray source.
A drawback of the known devices described above is that an apparent change in the sensitivity of the detector is liable to occur during an examination, thus introducing measurement errors which give rise to disturbing errors in the calculation of the density distribution of the examined section of the object. When an X-ray source in the form of, for example, a rotary anode X-ray tube is made, the location within the X-ray tube at which X-rays are generated (the X-ray focus) will change with respect to the envelope of the X-ray tube during an examination. This occurs because the mean temperature of the rotary anode may become as high as approximately 1500.degree. C. during an examination. An examination may have a duration of up to 30 seconds, and the shape and dimensions of the rotary anode and of the support thereof can change due to thermal expansion during this time. Vibration can also cause movement of the X-ray focus in an X-ray source.
Due to the movement of the X-ray focus in the X-ray source, a varying amount of radiation will be passed by the collimator via the space between adjacent collimator plates. This is because the collimator plates of the scattered radiation collimator are directed towards a predetermined location of the X-ray source. Accordingly, an apparent change will occur in the sensitivity of the detector situated behind the collimator.